The present invention relates to a device for estimating the loss ratio of data cells in a digital transmission network switch. It applies in particular to the construction of local area networks operating in the transfer mode known by the abbreviation ATM for xe2x80x9cAsynchronous Transfer Mode such as described in European Patent Application No. 0 0650 276 A1xe2x80x9d.
In a network operating in ATM mode, the digital information is exchanged between nodes of the network in the form of fixed-format packets of bits called xe2x80x9ccellsxe2x80x9d. Each cell is composed, on the one hand, of a header serving to identify the cell and the communication to which it belongs and, on the other hand, of an information field containing the useful data to be conveyed. The nodes of the network are responsible for routing the ATM cells applied to their inputs to one or more outputs. These nodes consist of switches, structured for example around a data transfer bus coupled to incoming highways and outgoing highways by way of shift registers organized as a stack, so as to store the temporarily surplus cells when several cells applied to different incoming highways are intended to be broadcast on the same output. However, the limited size of the shift register stacks incurs a risk of the loss of cells through overflow. Since the loss ratio of cells is directly dependent on the size of the stacks of registers, the latter are dimensioned so as to guarantee excellent loss values, of the order of 10xe2x88x9210 for example. However, regardless of this size, it is always possible for the register stacks to overflow, causing the loss of the relevant cells. Supervision of this phenomenon can consist in counting the number of cells actually lost, and in then deducing the loss ratio from this by comparing with the number of cells actually transmitted. However, the smallness of the ratios to be measured means that the confidence interval of the statistical estimate is too large for it to be easily coped with.
Another method consists in likening each of the traffic flows of cells which have passed into the switch to a more or less simple statistical model for which it is possible to calculate the multiplexing characteristics in an ATM queue.
The method introduced is novel in that it uses the actual traffic directly to estimate the occupancy of the stacks of output registers. The assumption which is made regarding the profile of the sources is minimal since it amounts simply to assuming that the traffic which has flowed can be estimated, the sources being regarded as stationary and ergodic.
The basis of the method resides in the existence of a scaling relation in the formula giving the loss ratio for a register of finite size B whose flow capacity is c cells per second and which is subjected to the traffic generated by N identical connections. This relation which is known in particular from the article in the IEEE Transactions on Communications Vol. 43 no. 2/3/4C. Courcoubetis et al entitled xe2x80x9cAdmission Control and Routing ATM networks using Inferences from Measured Buffer Occupancyxe2x80x9d is as follows:                               f          ⁡                      (                                                            (                                      1                    +                    ϵ                                    )                                ⁢                N                            ,              B              ,              c                        )                          =                  F          ⁡                      (                          N              ,                              B                ⁢                                  xe2x80x83                                ⁢                                  c                                      (                                          1                      +                      ϵ                                        )                                                                        )                                              (        1        )            
Estimation of the second term in relation (1) is possible by emulating the register stack corresponding thereto. This amounts to incrementing a counter at the rate of arrival of the cells and in decrementing it at the service rate c/(1+xcex5).
The function F is approximated by the relation                               F          ⁡                      (                          N              ,              B              ,                              c                                  1                  +                  ϵ                                                      )                          =                              A            ·            B                    -                      ϵ            ⁢                          xe2x80x83                        ⁢                          exp              ⁡                              (                                                      -                    B                                    ·                                      I                    ⁡                                          (                                              N                        ,                                                  c                                                      (                                                          1                              +                              ϵ                                                        )                                                                                              )                                                                      )                                                                        (        2        )            
in which the coefficients A,xcex5 and xcex5 are determined by giving B three different values of sizes of registers and by solving the corresponding equation system with three unknowns.
The error ratio estimated by this method is of the same order of magnitude as the actual loss ratio, however these good results are obtained only by virtue of good knowledge of the current state of the link traversed, of the profile of the calling traffic and regarding the quality objective demanded by all connections established and to be established, given that in an ATM network the decision to admit or to reject a connection may have weighty consequences, since any excess traffic is a potential source of degradation of the quality of the service offered to the connections already established.
The purpose of the invention is to alleviate the aforementioned drawbacks via a method of processing which demands no assumption regarding the statistical nature of the traffic.
To this end, the subject of the invention is a device for estimating the loss ratio of data cells in a digital transmission network switch comprising means for routing cells present on incoming highways to outgoing highways through stacks of registers, characterized in that it comprises a traffic summator for summing the number of cells which are present on each incoming highway during each cell instant and are destined for one and the same outgoing highway, a highway emulation group allocated to each outgoing highway and composed of a specified number of counters simultaneously loaded with the total number of cells which are destined for this highway and are presented on the incoming highways of the switch, the respective counts of each counter being limited by thresholds so as to estimate, when the number of cells totalled up in a counter exceeds a threshold, the number of cells which may be lost on the corresponding outgoing highway.